Using files

Table of contents

Using files

C provides functions through the <stdio.h> library for reading data from files and writing data to files.

The usual sequence is:

Open the file.
Check that it opened successfully.
Read from it or write to it.
Close it when finished.

Opening a file

The fopen() function opens a file:

FILE *file = fopen("data.txt", "r");

The first argument is the path to the file:

"data.txt"

The second argument is the mode, which tells C how we want to use the file:

"r"

FILE * is the type used by the C library to keep track of an open file. The * means that it is a pointer, which we will cover properly in Unit 1.4. For now, we can treat FILE * as a special data type used for an open file.

Checking that the file opened

Opening a file can fail, as the file might not exist or the path might be incorrect.

If fopen() cannot open the file, it returns NULL.

To avoid weird bugs, always check the result before trying to use the file:

FILE *file = fopen("data.txt", "r");

if (file == NULL) {
    printf("Could not open data.txt\n");
    return 1;
}

File modes

The mode supplied to fopen() controls how the file is opened.

Use "r" to open an existing file for reading:

FILE *file = fopen("data.txt", "r");

The file must already exist.

Use "w" to open a file for writing:

FILE *file = fopen("results.txt", "w");

If the file does not exist, it is created.

Be careful! If the file already exists, opening it with "w" removes its current contents. I have forgotten this and lost important stuff!

Use "a" to append data to the end of a file:

FILE *file = fopen("log.txt", "a");

If the file already exists, its contents are kept and the new output is added at the end.

You can also add b for binary files like "rb" or "wb" but we will stick with text files for now.

Writing to a file

The fprintf() function works like printf(), except that its first argument specifies the file:

fprintf(file, "Hello!\n");

We can use the usual format specifiers:

int score = 42;

fprintf(file, "Score: %d\n", score);

The text is written to the file instead of the terminal.

Reading from a file

We can use our friend fgets() to reads one line from a file, just like we did with the string examples:

char line[100];

fgets(line, sizeof(line), file);

Its arguments are:

the character array where the line will be stored
the size of that array
the file to read from

If a line is read successfully, fgets() returns a non-NULL value.

If there is nothing left to read, or an error occurs, it returns NULL.

This means we can use it as the condition of a while loop:

while (fgets(line, sizeof(line), file) != NULL) {
    printf("%s", line);
}

The loop stops when there are no more lines to read.

fgets() normally keeps the newline from the file, so we use:

printf("%s", line);

rather than adding another \n.

Converting a line into a number

A line read by fgets() is a string, even if it contains a number.

For a simple example, we can use sscanf() to extract a floating-point value:

float value;

if (sscanf(line, "%f", &value) == 1) {
    printf("Value: %.2f\n", value);
}

The return value tells us whether one value was successfully converted.

More detailed input conversion and validation are covered on the User Input Handling page.

Closing a file

Every file opened using fopen() should be closed using fclose():

fclose(file);

Closing the file finishes any pending writes and releases the resources used by the file.

Do not continue using the FILE * after the file has been closed.

Complete example

This program reads numbers from numbers.txt, calculates their average and writes the result to stats.txt.

The input file contains one number on each line:

12.5
3.0
7.25
-1.5
10

The program is:

#include <stdio.h>

#define MAX_LINE_LENGTH 100

int main(void)
{
    // Open the input file
    FILE *input = fopen("numbers.txt", "r");

    if (input == NULL) {
        printf("Could not open numbers.txt\n");
        return 1;
    }

    // Open the output file
    FILE *output = fopen("stats.txt", "w");

    if (output == NULL) {
        printf("Could not open stats.txt\n");
        fclose(input);
        return 1;
    }

    char line[MAX_LINE_LENGTH];
    float sum = 0.0f;
    int count = 0;

    // Read and process each line
    while (fgets(line, sizeof(line), input) != NULL) {
        float value;

        if (sscanf(line, "%f", &value) == 1) {
            sum = sum + value;
            count = count + 1;
        }
    }

    // Make sure at least one number was read
    if (count == 0) {
        printf("No numbers were found\n");

        fclose(input);
        fclose(output);

        return 1;
    }

    float average = sum / count;

    // Write the result
    fprintf(output, "statistic,value\n");
    fprintf(output, "average,%.2f\n", average);

    // Close both files
    fclose(input);
    fclose(output);

    printf("Average written to stats.txt\n");

    return 0;
}

The output file contains:

statistic,value
average,6.25

The main things to notice are:

both files are checked after opening - always do this ;)
each line is read using fgets()
sscanf() converts the line into a number
the average is only calculated if at least one number was found
both files are closed before the program finishes

A useful pattern to remember is:

open → check → use → close